The special Steam Card webpage provides very few details on the vouchers. But maybe details aren’t really needed. You simply buy them, and then use the enclosed keycode to fill up your Steam Wallet (on your PC, of course) with lovely monies. Apparently there will be a $20 card, as well as a $50 card, and they will presumably cost as much as the Steam Wallet dollars offered within. Hopefully this will spice up the Steam trading economy. More details over at Joystiq.